Low-pressure steam turbine

ABSTRACT

A low-pressure turbine is provided with guide vanes and rotating vanes, the vane blades of which have a coating that is hydrophobic or water-repellant and has a smooth surface. The coating preferably contains amorphous carbon. The hydrophobic property of the coating has the result that small droplets contained in the steam phase, upon impacting a coated vane blade, roll off across the vane blade in the form of small droplets and further follow the steam flow. This prevents moisture losses and increases the efficiency of the turbine. It also prevents the small droplets on the vane blades from coalescing into larger drops or into a fluid film. This prevents erosion due to the impact of large drops on vanes and other components of the turbine.

FIELD OF TECHNOLOGY

[0001] The invention relates to the vanes of a low-pressure steam turbine, and in particular to a coating for such vanes used to increase the efficiency of the low-pressure steam turbine.

STATE OF THE ART

[0002] The expansion of the turbine steam in a low-pressure turbine to condenser pressure usually results in a range for wet steam. The mass content of the condensation water in the wet waste steam can be up to 14%. The impulse of the entire mass stream of the turbine steam is preserved, independently from the condensation water content. However, the presence of a liquid phase in the rotating and stationary elements of the turbines results in increased dissipative losses. In a so-called low-pressure turbine, about 12-14% of the mass stream is generated in the form of water. This moisture loss results in a loss of efficiency of the low-pressure turbine of approximately 6-7%, which corresponds to a loss of efficiency of approximately 1-2% of an entire steam power plant. In combination and nuclear power plants, the contribution of power of the low-pressure turbine in relation to the overall plant power is slightly higher by comparison, so that the loss in overall efficiency due to moisture losses is approximately 2-2.3% or 3-3.5% overall.

[0003] The extent of losses depends for the most part on the size of the water drops. In most cases, only small drops, in the range of micrometers, are contained in the steam phase. According to the newest understanding, these drops maintain their size and do not coalesce into larger drops as long as they keep floating or flowing in the steam. Similar to a vapor, they flow along with the steam flow that exerts the impulse onto the vanes. As long as the drops remain in this small size range, they do not have an adverse effect on either the operation or on the performance of the turbine. However, as they flow through the guide vanes and rotating vanes, the drops grow. During the contact with metal surfaces, probably in particular with the concave metal surfaces of the guide vanes, the small condensate drops spread on the surface and form a closed condensate film that flows on the guide vanes over the concave or convex surfaces subject to the effect of the shearing forces of the steam. At the trailing edge of the guide vane, the fluid film leaves the surface and is hereby accelerated and divided by the rotating bucket wheel. The drops generated by this division have a larger diameter than the drops created by spontaneous condensation.

[0004] By centrifugal forces, these larger drops are spun outward by the rotating vanes in the direction towards the turbine housing. This means that a part of the impulse of the working medium is not transferred onto the vanes, which results in a moisture loss that reduces the degree of efficiency of the low-pressure turbine. This phenomenon is even stronger the more that the size and mass of the drops, and therefore of the centrifugal force, increase. Furthermore, accumulations of water at the inside surfaces of the housing of the low-pressure turbine result in dissipative friction losses on the rotating vane tips and vane covers. Finally, enlarged drops with diameters in the range from 100-200 m[sic] and speeds in the range of more than 250 m/s cause erosion due to the impact of the drops. This erosion depends greatly on the specific materials, whereby vane materials of titanium and titanium alloys, which are used preferably for the large vanes of the low-pressure turbine, are especially susceptible.

[0005] For a long time already, attempts have been made to provide the vane blades of turbines with a coating that would increase the erosion resistance of the vanes, thus extending their useful life. DE 37 24 626 describes a coating for the vane blades of a steam turbine that consists of a hard, wear-resistant ceramic material or of a multi-layer coating of active metal and a cover layer of a ceramic material. The coating made from ceramic material is used to increase the erosion resistance of the vane blades, while the layer of active material improves the adhesion of the coating.

DESCRIPTION OF THE INVENTION

[0006] Based on the initially described losses and damage encountered with low-pressure turbines, which are caused by condensate drops in the steam flow, the invention has the objective of creating vanes for a low-pressure turbine on whose metallic surfaces the formation of a closed fluid film is prevented, so that moisture losses are reduced and the efficiency of the turbine is increased, and at the same time resistance against erosion due to the impact of drops is ensured.

[0007] This objective is realized with a low-pressure turbine according to claim 1. According to the invention, at least the vane blades of the stationary guide vanes in a low-pressure turbine are provided with a coating that is hydrophobic or water-repellant, and has a smooth surface.

[0008] The hydrophobic property of the vane blade coating has the result that the smaller droplets contained in the steam phase maintain their identity when they impact a coated vane blade. It also prevents droplets from coalescing into larger drops, and prevents even a close fluid film from forming. Instead, all droplets roll across the coated surface. Once they roll off the vane, they remain floating droplets following this steam flow, and in this way exert an impulse on other vanes. In low-pressure turbines in which vanes are provided with the coating according to the invention, moisture losses are substantially prevented. This also means that the efficiency of the low-pressure turbine is increased.

[0009] The invention furthermore has the advantage that the hydrophobic coating indirectly prevents erosion due to the impact of drops as a result of large drops on the vane blades or other turbine components. As a result of the hydrophobic property of the coating, no large drops form on the vane blade, and no fluid film that would be divided into large drops at the vane edge is formed. This means that no large drops that would cause erosion due to the impact of drops are created. A smooth coating furthermore supports the quick rolling-off of the small droplets and helps prevent the coalescing into larger drops.

[0010] In a first embodiment of the invention, the hydrophobic coating contains amorphous carbon. In the following text, this shall mean to include hydrocarbon-containing carbon layers with up to 10 to 50 atom % hydrogen content and a ratio of sp³ to sp² bonds between 0.1 and 0.9. In general, all amorphous or dense carbon layers as well as plasma polymer layers, polymer-like, or dense carbon and hydrocarbon layers produced with carbon or hydrocarbon precursors can be used, as long as they have the hydrophobic mechanical or chemical properties mentioned below of the amorphous carbon for the production of individual layers or sequences of layers. Amorphous carbon, also called diamond-like carbon, is generally known for its extraordinary hardness, chemical stability, as well as elasticity. Under certain conditions, amorphous carbon furthermore has a low surface energy in comparison to the surface tension of water, so that a hydrophobic or water-repelling property is achieved. The hardness of amorphous carbon hereby can be changed by varying the parameters for producing a coating. In comparison to a hard layer, a soft layer (within the hardness range of amorphous carbon) should only be understood to be less hard. In particular, a soft or less hard layer has a distinct hydrophobic property.

[0011] For this reason, the coating in a preferred embodiment has a hydrophobic single layer with amorphous carbon or a plasma polymer with a thickness between 1 and 8 micrometers and a hardness between 500 and 1500 Vickers. It is preferred that the thickness is between 2 and 4 micrometers.

[0012] In addition, amorphous carbon or a plasma polymer adheres very well to typical vane materials, so that it is not necessary to ensure sufficient adhesion by roughening the vane material. Amorphous carbon or a plasma polymer is therefore particularly suitable for long-term applications, for example on vane blades in a turbine.

[0013] In another embodiment of the coating, the hydrophobic single layer is constructed with amorphous carbon or a plasma polymer as a gradient layer. The gradient layer has a hardness that gradually changes through its depth, whereby its lowest depth ranges are the hardest, and its top depth ranges are the softest. The top depth ranges of the gradient layer hereby have a hardness between 500 and 1500 Vickers and a thickness between 0.1 and 2 micrometers. The lower depth ranges of the hydrophobic gradient layer have a hardness between 1500 and 3000 Vickers and a thickness between 0.1 and 6 micrometers, preferably between 1 and 3 micrometers.

[0014] In another embodiment, the hydrophobic coating again contains amorphous carbon or a plasma polymer. However, the coating in particular has a discrete sequence of layers with at least one hard layer with amorphous carbon and at least one soft layer with amorphous carbon or a plasma polymer, whereby the hard and soft layers are applied alternately to the surface of the vane blades. The first layer hereby is a hard layer, and the last one a soft layer, whereby the soft layer is similar to the coating in the first embodiment and has a hydrophobic property.

[0015] The coating according to the invention has the additional advantage that it offers protection from erosion due to the impact of drops. As already mentioned, because larger drops are prevented from forming, on the one hand, fewer drops of a size that could cause erosion due to drop impact on vane blades, vane covers, and housing components, etc., are present. On the other hand, the second embodiment's sequence of layers of hard and soft (or less hard) layers ensures additional protection from erosion due to drop impact since it absorbs the impulse of an impacting drop. The impulse of impacting drops is absorbed by the soft and hard layers in that the compression waves originating from the impact of the drops are extinguished by interference by the pairs of hard and soft layers. The extinction of the compression waves is similar to the extinction of optical waves caused by pairs of thin layers with respectively high and low indices of refraction. The extinction of compression waves is further increased by a layer sequence of several layer pairs of hard and soft layers. In another embodiment, the coating according to the invention has several pairs of layers with one each hard and soft layer of amorphous carbon.

[0016] In another embodiment of the invention, not only the vane blades of the guide vanes but also the vane blades of the rotating vanes are provided with the coating according to the invention. Here the formation of a film of condensate on the surfaces of both the guide as well as the rotating vanes is prevented. This further ensures an increase of the efficiency of the low-pressure turbine by preserving the condensate in the form of small droplets in the steam flow. The rotating vanes are additionally protected from erosion due to the impact of drops.

[0017] The coating according to the invention can be realized according to various generally known production processes, for example by precipitation using corona discharge in a plasma of hydrocarbon-containing precursors, ion beam coating, and sputtering of carbon in a hydrogen-containing working gas.

[0018] In these processes, the substrate is exposed to a stream of ions with several 100 eV. During corona discharge, the substrate is arranged in a reactor chamber in contact with a cathode connected capacitatively with a 13.56 MHz HF generator. The grounded walls of the plasma chamber hereby form a large counter-electrode. In this arrangement, any hydrocarbon vapor or hydrocarbon gas can be used as a first working gas for the coating. In order to achieve special layer properties, for example different surface energies, hardness values, optical properties, etc., different gases are added to the first working gas. By adding nitrogen, fluorine- or silicon-containing gases, high or low surface energies are achieved, for example. Added nitrogen always increases the hardness of the resulting layer. By changing the bias voltage above the electrodes between 100 and 1000 V furthermore makes it possible to control the resulting hardness of the layer, whereby a high bias voltage produces a hard, amorphous carbon layer, and a low voltage produces a soft, amorphous carbon layer. In order to achieve a gradient layer, the production parameters, such as, for example, the composition of the plasma in the reactor chamber, are gradually changed.

[0019] In one exemplary embodiment of the invention, the hardness of a hard layer of a pair of layers is between 1500 and 3000 Vickers, while the hardness of a soft layer of a pair of layers is between 800 and 1500 Vickers.

[0020] The thicknesses of the individual layers hereby are between 0.1 and 2 m[sic], preferably between 0.2 and 0.8 m[sic], if several layers are applied consecutively in the sequence of layers.

[0021] Furthermore, the thickness values of the harder layers and softer layers are preferably inversely proportional to their hardness values.

[0022] The adhesion of the coating according to the invention is well ensured for most types of substrates. It is especially good for materials that form carbides, such as titanium, iron, and silicon, as well as aluminum, but not for precious metals, copper, or copper-nickel alloys. The substrate surface, i.e. the surface of the vane blade, hereby does not require roughening. To further improve adhesion, an adhesion layer, on which the sequence of layers according to the invention is then applied, could be realized on the surface of the vane blade. A suitable adhesion layer is, for example, titanium.

[0023] The coating according to the invention therefore can be applied to different substrate materials used for vanes, such as, for example, titanium, stainless steel types, chrome steel types, aluminum, as well as all materials forming carbides.

[0024] The coating according to the invention is particularly suitable for increasing the efficiency of existing low-pressure turbines (retrofit), since the vanes can be removed, coated, and then without any further work, reinstalled. 

1. Low-pressure turbine with vanes comprising stationary guide vanes and rotating vanes characterized in that at least the vane blades of the guide vanes are provided with a coating that is hydrophobic and has a smooth surface.
 2. Low-pressure turbine as claimed in claim 1, characterized in that the hydrophobic coating contains amorphous carbon or a plasma polymer.
 3. Low-pressure turbine as claimed in claim 2, characterized in that the hydrophobic coating is a single layer with a thickness between 1 and 8 micrometers.
 4. Low-pressure turbine as claimed in claim 3, characterized in that the hydrophobic single layer has a hardness between 500 and 1500 Vickers.
 5. Low-pressure turbine as claimed in claim 2, characterized in that the hydrophobic coating is a single layer that is applied as a gradient layer.
 6. Low-pressure turbine as claimed in claim 5, characterized in that the top depth ranges of the gradient layer are relatively soft and have a hardness between 500 and 1500 Vickers.
 7. Low-pressure turbine as claimed in claim 5, characterized in that the top depth layers of the gradient layer have a thickness between 0.1 and 2 micrometers.
 8. Low-pressure turbine as claimed in claim 5, characterized in that the bottom depth ranges of the hydrophobic single layer have a hardness between 1500 and 3000 Vickers and a thickness between 0.1 and 6 micrometers.
 9. Low-pressure turbine as claimed in claim 2, characterized in that the coating has a discrete sequence of layers with at least one hard layer with amorphous carbon and at least one soft layer with amorphous carbon or a plasma polymer, and where at least one hard and at least one soft layer are applied alternately to the surfaces of the vane blades, and the lowest layer of the sequence of layers is a hard layer with amorphous carbon or a plasma polymer, and the last layer of the sequence of layers is a soft layer with amorphous carbon or a plasma polymer, and at least the last, soft layer has a hydrophobic property.
 10. Low-pressure turbine as claimed in claim 9, characterized in that the hard layers with amorphous carbon or a plasma polymer each have a hardness between 1500 and 3000 Vickers, and the soft layers with amorphous carbon or a plasma polymer have a hardness between 500 and 1500 Vickers.
 11. Low-pressure turbine as claimed in claim 9, characterized in that the hard and soft layers of the discrete sequence of layers each have a thickness between 0.1 and 2 micrometers.
 12. Low-pressure turbine as claimed in claim 9, characterized in that the thicknesses of the hard and soft layers are inversely proportional to their hardness.
 13. Low-pressure turbine as claimed in one of the previous claims characterized in that an adhesion layer is applied between the surface of the vane blades and the coating.
 14. Low-pressure turbine as claimed in one of the previous claims characterized in that the coating is applied to the surfaces of the vane blades of the stationary guide vanes and the vane blades of the rotating vanes of the low-pressure turbine. 